1. Field of the Invention
The present invention generally relates to a vibration damper, in particular for motor vehicles, such a vibration damper including a work cylinder, a work piston fastened to a piston rod and dividing the work cylinder into two work chambers, and possibly an equalization chamber and a piston rod guide which seals the piston rod against the atmosphere, whereby the work chambers are filled with a magnetorheological fluid which can be acted upon by an electric coil.
2. Background Information
Such known vibration dampers are disclosed, for example, in U.S. Pat. No. 5,277,281, in which the work chambers of the work cylinder are separated by a work piston and are filled with a magnetorheological fluid. The work piston thereby has flow passages through which the fluid flows and generates a corresponding damping force. Such a fluid essentially changes its viscosity when a magnetic field is applied to it, so that the damping force of the vibration damper is variable, i.e. it can be adjusted in a variable manner. The change of the viscosity is generated by an electric coil and a soft iron circuit and the related magnetic field strength. If no current flows through the coil, the magnetorheological fluid acts like a normal liquid with a relatively low viscosity. Only when a current is applied to it by the electric coil does the viscosity of the fluid increase in the magnetic field. One disadvantage, therefore, is that when a malfunction or defect occurs, e.g. a power failure or a break in a cable, the damping force of the vibration damper is relatively low, which behavior can be undesirable.